Cancer development relies on the dynamic balance between cancer epithelial cells and the tumor vasculature. Thus, antiangiogenic therapy is a rational and effective approach against tumor growth and progression. Vascular endothelial growth factor A (VEGFA or VEGF) is a well-proven dominant growth factor in tumor angiogenesis, which has been further substantiated by the clinical promise of targeting VEGF in the treatment of human cancers. The HIF-1 transcription factor complex (HIF-1a/HIF-1B) plays an essential role in the response of cancer cells to hypoxia. Extensive evidence shows that HIF-1 induces the expression of genes important in hypoxia response such as anti-apoptosis, glycolysis, angiogenesis, cell migration, and metastasis. However, it remains unclear whether HIF-1a solely mediates tumor survival under hypoxia; or whether redundant pathways contribute to mechanisms of survival. For example, the HIF-2 complex (HIF-2a/HIF-1B) may also regulate genes important in hypoxia response. A growing concern with effective antiangiogenic therapy is tumor reactive resistance: The resulting increase in intratumoral hypoxia may drive the selection of cells that are less dependent on the vasculature, cells which in turn may be more malignant. While reactive resistance is thought to be driven by hypoxia, it remains to be determined whether reactive resistance is mediated by HIFs (HIF-1a and HIF-2B). The primary aim of this proposal is to determine the direct role of HIF-1a in mediating tumor response to antiangiogenic therapy. The secondary aim is to distinguish the roles of HIF-1a and HIF-2B in tumor response to hypoxic stress. To directly investigate these aims, a pre-clinical model system, using human cancer cell knockout technology, was developed. The insights gained from these studies should contribute to our understanding of cancer biology and the rational combination of antiangiogenic therapies.